Don't give yourself a rating on the USTA site until you've played a few players on possible teams to ensure your guess is correct in your region. If you accidently rate yourself low you can play up in leagues and your rating will follow. If you accidently over rank yourself you'll have trouble finding a team willing to play you enough to get your rating knocked back down.

Play some matches that are unofficial and unrecorded first. You may have the skills of a 4.0 and still lose to 3.0-3.5 players, especially pushers, if you don't have match experience. Then do what Phil said.

Sign up with the USTA and go with a 3.5 rating, and start playing local tourneys at the 3.5 level and see how you do. You can always play 4.0 level if you are 3.5 but can't play 3.5 if you sign up as 4.0. You can find local tourneys on the USTA.com website. Start your own weekly hitting group if you know enough people. Then everyone can improve and you will have good hitting partners. Get on a USTA league team if you know a club captain. Good luck and have fun.